This study describes the impact of septic tanks on the groundwater quality of the shallow Pleistocene
aquifer in Jericho area, Westbank, Palestine. Septic tanks are widely used for storage
and disposal of sewage in the populated and agricultural city of Jericho. Routine hydrochemical
tests for groundwater quality performed for several years identified the problem of a gradual
nitrate increase, without pinpointing its definite sources. The geological formations of the Jericho
area and the shallow nature of the Pleistocene aquifer, together with the mechanism of
recharge, make the groundwater in this aquifer highly susceptible to contamination, particularly
along sewers. The lithology of the Samara (high hydraulic conductivity) and the Lisan formation
(low conductivity but increased infiltration along fractures) promote easy seepage of
agricultural and anthropogenic inputs into the groundwater. Nitrate concentrations are elevated
near septic tanks and animal farms, with nitrate values exceeding 74 mg/L. d15Nnitrate and
d18Onitrate signatures suggest sewage and manure as the main sources of high nitrate concentration
in the groundwater. Samples taken during the end of the dry season indicate that a slight
denitrification in the aquifer